TP: Phase Diagrams

Phase diagrams

A pure substance cannot occupy any arbitrary point on the P,T, v space, instead being confined to a surface (i.e., any two that are identified specify the third).

For this reason, one often visualizes the phase behavior of a species by taking a projection of the PvT spaces in two dimensiones: typically in the P-v or P-T plan.


A PT diagram shows the phase behavior of a single species in the pressure-temperature plane. Here, lines denote the boundaries between phases (or the points where the phases coexist).

PT diagram


The saturation pressure is the coexistence pressure between liquid and vapor for a pure species and is a function of temperature (only). (see state postulate)


While the vapor pressure is similar to the saturation pressure, the vapor pressure refers to the pressure in a mixture of components (i.e., the total pressure is higher).


Describe the difference between the saturation and vapor pressures

Pv diagram

Coexistence on a Pv diagram is shown on the liquid-vapor dome, since v is very different for liquids and vapors (up to the critical point!).

Coexistence on a Pv diagram is shown on the liquid-vapor dome.


A supercritical fluid is a substance which is above its critical temperature, Tc. Here there is no vapor-liquid transition so the liquid-vapor dome ends.

  • Saturated essentially means "in phase equilibrium"
  • Superheated means at a temperature above the saturation/coexistence curve
  • Subcooled means at a temperature below the saturation/coexistence curve
  • Appendix B1, B2

On the vapor-liquid line the temperature is called the boiling point and the pressure is the vapor pressure.

On the vapor-solid line the temperature is called the sublimation point.

On the liquid-solid line the temperature is called the freezing/melting point.


Identify the phases present on a PT and/or Pv diagram as well as the critical point and triple point